Scraps or Metal Scrap Parts are used in the Game to Upgrade the Old Train's weapon damage, shield, and speed. The player also uses one scrap to heal 10% if their train's health, this can be done mid battle if needed. If some person on the Island wants something done by the Player, they will offer them Scrap as payment (Mostly for optional missions). You can Upgrade the Old Train by using these Scraps. Scraps can also be found on the ground and in large yellow crates across the world.
Scraps do not respawn around the map, only on large yellow crates.
"Metal Scraps that can be used to upgrade your train. Use the blueprints in your train to access the upgrades menu"
(Metal Scrap's Description)